Encryption of transmission data is performed as a security measure. Shared-key cryptography (also called symmetric-key cryptography) is one of the methods for encryption. Meanwhile, to further enhance security, there is a technique of changing the shared key at predetermined time intervals, as the well-known art described below.
In addition, there are security systems such as WEP (Wired Equivalent Privacy) and WPA (Wi-Fi Protected Access).
According to these techniques, the authentication process is performed generally by issuing a control instruction in a server.
Meanwhile, in a communication system, a technique to change the common cryptographic key by changing only the control variable of the server while keeping the security code at the client side unchanged has been disclosed (for example, Japanese Laid-open Patent Publication No. 9-321748). This makes it possible to improve the security of a cryptographic system by changing the common shared key at short time intervals.